Vinyl chloride-olefin copolymers having good color stability and flexibility for container coatings

ABSTRACT

Disclosed herein is an improved vinyl chloride latex copolymer composition having good color stability, characterized in that it contains a small but effective amount of an alpha-olefin to provide color stability of films of the composition.

This is a divisional of application Ser. No. 07/328,990, filed Mar. 27,1989 now U.S. Pat. No. 4,948,834.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to vinyl chloride latex copolymers. Morespecifically, the present invention relates to vinyl chloride copolymersemploying small but effective amounts of ethylene comonomer.

2. Brief-Description of the Prior Art

Vinyl chloride latex copolymers are generally known in the art. Thesecopolymers are advantaged by color, clarity, flexibility, and barrierproperties. These properties make the vinyl chloride latex copolymerswell suited to use in plastics and surface coatings. Premature failureof some of the above properties under processing or service conditionsof products containing the vinyl chloride latex copolymers is, however,of concern. Of particular concern here is the problem of discolorationof coatings containing vinyl chloride latex copolymers upon baking athigh temperatures of about 350° F. or higher. In order to avoid orprevent discoloration at the bake temperatures, stabilizers, such asorganotin compounds and certain metal carboxylates, are often added tothe coating.

By the present invention, there is provided a novel class of vinyl latexcopolymers wherein discoloration at bake temperatures is prevented orreduced. Notably, the new class of vinyl latex copolymers display yetother improved properties, which are discussed more fully herein.

SUMMARY OF THE INVENTION

In accordance with the foregoing, the present invention encompasses animproved vinyl halide latex copolymer composition having colorstability, containing a latex copolymer of free-radical additionpolymerizable ethylenically unsaturated monomers in water in thepresence of a polymeric surfactant; the improvement comprising acopolymerizable alpha-olefin monomer selected from the group consistingof ethylene, propylene, isobutylene, and butene-1 which is present in asmall but effective amount to reduce or prevent discoloration of a filmof the vinyl halide latex copolymer composition, upon baking at atemperature of about 350° F. or higher. In the present embodiment of theinvention, the vinyl halide monomer is vinyl chloride, and thealpha-olefin is ethylene in amounts of about 1 to 4 percent by weightsolids based on the total weight of the copolymerized monomers of thelatex copolymer. The term "latex copolymer" is used to refer to thecopolymerized monomers in the dispersed phase. The term "latex copolymercomposition" is used to refer to the final composition comprising thelatex copolymer and polymeric surfactant(s).

The invention further encompasses a coating composition, particularly awater-based coating composition containing the vinyl halide latexcopolymer composition of this invention and a method of using the sameto reduce discoloration of coated substrates. Also encompassed by theinvention is an article of matter which is prepared by applying thecoating composition of this invention to a substrate such as an aluminumsubstrate.

The article of matter of this invention exhibits reduction or preventionof discoloration when a film of the coating composition is baked at atemperature of about 350° F. or higher. In the preferred embodiment, thebaked coatings are notably coalesced and, of course, have the additionalproperties, principal among which is freedom from taste. Hence, thecoating compositions of this invention are very well suited to use asinterior can coatings.

DETAILED DESCRIPTION OF THE INVENTION

The vinyl halide latex copolymer of this invention is principally of avinyl halide and an alpha-olefin content wherein the alpha-olefin ischaracterized in that it is present in small but effective amounts toreduce or prevent discoloration of a film of the latex copolymercomposition. The alpha-olefin content can be from about 0.5 to 6 percentand preferably about 1 to 4 percent by weight solids based on themonomer content of the latex copolymer. The vinyl halide content of thecopolymer can be from about 75 to 99.5 percent and preferably about 90to 99 percent by weight solids based on the copolymerized monomercontent of the latex copolymer.

In a preferred embodiment of the invention, the vinyl halide latexcopolymer further contains a copolymerized epoxy-containing monomer. Anexample of said monomer can be glycidyl acrylate, glycidyl methacrylate,allyl glycidyl ether or the like. The epoxy-containing monomer can beemployed in amounts of about 0.5 to 20 percent and preferably about 0.5to 4 percent by weight solids of the copolymerized monomer content ofthe latex copolymer.

In the process of this invention, the latex copolymer composition can beprepared by a free-radical initiated addition polymerization of themonomers described hereinabove in water in the presence of a surfactant,which is preferably a polymeric surfactant such as a base-neutralizedacid-functional addition polymer.

It is a distinct feature of the invention that relatively low amounts ofalpha-olefin are employed herein. Copolymerization reactions ofalpha-olefin, such as ethylene, is usually performed under pressure. Aswould be realized, when a low amount of ethylene is employed, theapplied pressure is easily manageable. Hence, there is provided herebyan improved process for preparing a copolymer of vinyl halide at areduced pressure.

It is also a distinct feature of the invention that substantially highamounts of the vinyl halide monomer can be employed herein. By usinghigh levels of, say, vinyl chloride, the benefits associated with thismonomer can be maximized. The cost of the resultant copolymer can bereduced since the vinyl chloride monomer is relatively inexpensive.

The method of preparing the latex copolymer composition is describedhereinbelow. The preferred polymeric surfactant, which is useful herein,can be a base-neutralized acid-functional acrylic polymer. The base-neutralized acid-functional acrylic polymer is prepared by free-radicaladdition polymerization of ethylenically unsaturated monomers at leastone of which is an acid-functional monomer which is preferably anacrylic acid or methacrylic acid. Other acids or equivalents thereofthat are useful herein can be itaconic acid, maleic acid, fumaric acid,monoesters of unsaturated dicarboxylic acids, e.g., methyl hydrogenmaleate and ethyl hydrogen fumarate as well as anhydrides thereof wherethey exist, can also be used. Other monomers can be copolymerized hereinprovided that the copolymerizable monomer does not adversely affect thepreparation or use of the final product.

The acid-functional monomer is employed in an amount of about 40 to 100percent and preferably about 50 to 90 percent by weight based on thetotal monomer content of the acid-functional acrylic polymer. Themolecular weight (Mw) of the acid-functional addition polymer can befrom about 3,000 to 100,000 and preferably from about 10,000 to 80,000based on polystyrene standards.

The acid-functional addition polymer is neutralized with a base which ispreferably a fugitive base. The degree of neutralization can be fromabout 0 to 100 and preferably about 30 to 80 percent. Suitable basesinclude ammonia and primary, secondary or tertiary amines, e.g.,ethanolamine, diethanolamine, N-methylethanolamine,dimethylethanolamine, isopropanolamine, diisopropanolamine, methylamine,ethylamine, diethylamine, trimethylamine, triethylamine and morpholine.

The latex copolymer composition can be prepared by free-radicalinitiated addition polymerization of the vinyl monomer and alpha-olefinin the presence of a surfactant. On a percent basis, thebase-neutralized acid-functional polymeric surfactant can be employed inamounts of about 5 to 90 percent and preferably about 5 to 55 percentbased on total resin solids of the latex copolymer composition. Thepolymeric surfactant can be used in combination with conventionalsurfactants such as ionic or nonionic surfactants. As aforestated, thevinyl monomer, alpha-olefin monomer can be copolymerized with anepoxy-containing monomer. Yet, other copolymerizable monomers can beemployed herein provided that the preparation and use of the latexcopolymer is not adversely affected.

Thermal or redox free-radical initiators which can be used in thepreparation of the latex copolymer can be water-soluble or oil-solubleinitiators such as peroxide-types, e.g., hydrogen peroxide, t-butylhydroperoxides/isoascorbic acid; ionic types, e.g., ammonium, sodium orpotassium persulfates or azo types, e.g.,azobisisovaleronitrile;azobis(N,N'dimethyleneisobutyl)amidene. Typically, there are employed inamounts of about 0.01 to 3 percent and preferably about 0.05 to 1percent of the initiator based on the weight of the copolymerizablemonomers.

The polymerization temperatures employed herein can be from about 25° C.to 90° C. and preferably from about 40° C. to 75° C. Depending on howmuch ethylene is added herein, the reactor pressure can be from about 80to 400 pounds per square inch gravity (psig) and preferably from about170 to 350 psig. The resultant latex copolymer composition has a solidscontent of about 20 percent or higher and typically up to about 40percent and particle size of 500 to 5,000 angstroms and viscosity ofabout 10 to 10,000 centipoises.

In the practice of this invention, the latex copolymer composition canbe employed in coating compositions in an amount sufficient for theintended use. In interior can coatings, for example, the latex copolymercomposition should be present in an amount sufficient to provide barrierproperties and other desirable fabrication and durability properties.The coating composition can contain curing agents such as aminoplasts,phenoplasts or isocyanates. The curing agent is employed in amountssufficient to produce the requisite degree of crosslinking.

Aminoplast resins are the condensation products of an aldehyde, e.g.,formaldehyde, acetaldehyde, crotonaldehyde, and benzaldehyde with anamino or amide group-containing substance, e.g., urea, melamine, andbenzoguanamine. Products obtained from the reaction of alcohols andformaldehyde with melamine, urea or benzoguanamine are preferred in theaqueous-based coating compositions because of their good waterdispersibility. Useful alcohols used to make the etherified products arethe monohydric alcohols, such as methanol, ethanol, propanol, butanol,hexanol, benzyl alcohol, cyclohexanol, and ethoxyethanol. An etherifiedmelamine-formaldehyde resin is the preferred aminoplast resin. U.S. Pat.No. 4,075,141, Porter et al, Feb. 21, 1978, contains a description ofuseful aminoplast resins and is incorporated herein by reference.

Phenolic resins include the condensation product of an aldehyde with aphenol. Formaldehyde and acetaldehyde are preferred aldehydes. Variousphenols can be used, e.g., phenol per se, reseol, p-phenylphenol,p-tert-butylphenol, p-tert-amylphenol and cyclopentylphenol. Themethylolphenol ethers described in U.S. Pat. No. 2,597,333 (hereinincorporated by reference) are especially useful.

A number of blocked polyisocyanates are satisfactory crosslinkingagents. These agents are well known in the art. Generally, the organicpolyisocyanates are blocked with a volatile alcohol,epsilon-caprolactas, or ketoxime. These blocked polyisocyanates becomeunblocked at elevated temperatures, e.g., above about 100° C. "TheChemistry of Organic" Film Formers", Robert E. Kreiger Pub. Co.,copyrighted 1977, by D. H. Solomon, pages 216-217, contains adescription of many blocked isocyanates that can be used here. Thedisclosure of this publication is herein incorporated by reference.

Also contained in the coating compositions can be additives such as flowcontrol agents, adhesion promoters, cure catalysts and the like. Thecoating compositions of this invention can be prepared by blending thelatex copolymer, the curing and other coating additives, and adjunctsthoroughly. The coatings can be cured by baking at temperatures of 300°F. to 500° F. to commercial quality coatings having color stability.

The coating composition can be applied to substrates such as aluminum orsteel by spraying, roll coating, and electrodeposition method. Thearticle of matter comprising the cured coated substrates arecharacterized by the properties, principal among which are colorstability barrier properties, adhesion, and impact resistance.

These and other properties are more fully illustrated by the followingnon-limiting examples.

EXAMPLE 1

This example illustrates the vinyl chloride latex copolymer compositionsof this invention and a method of preparing and using the same.

    ______________________________________                                        Ingredients         Parts by Weight (grams)                                   ______________________________________                                        Reactor charge                                                                Acid-functional acrylic                                                                           623.0                                                     polymer surfactant.sup.1                                                      Ammonium persulfate         4.4                                               Deionized water      mix    50.0                                              Deionized water             2090.0                                            Monomer Charge (MC).sup.2                                                     Vinyl chloride      915.0                                                     Glycidyl methacrylate                                                                             20.0                                                      ______________________________________                                         .sup.1 Acrylic acid, 40 percent; styrene, 59 percent; ethyl acrylate, 1       percent on a weight basis.                                                    .sup.2 Prepared by blowing the glycidyl methacrylate into the vinyl           chloride over five minutes at 100 revolutions per minute (rpm).          

The polymerization was conducted in a sealed reactor equipped with anagitator, a means for heating, cooling and a means for purging with aninert gas.

The reactor charge was added to the reactor which was then subjected tovacuum. The reactor charge was then subjected to stirring at 300 rpm.One hundred ten grams of the monomer charge was added to the reactorcharge with stirring at 300 rpm at amaximum rate and at a pressure of 40psig at room temperature. An addition of ethylene to the reactor chargewas commenced and continued until a pressure of 219.4 psig was achievedat room temperature; the reactor charge was being stirred at 300 rpm.The reactor charge was then heated to 75° C, a pressure of 259.5 psigwas attained at that temperature. The reaction mixture was held at 75°C. for about one hour until the pressure began to drop. After one hour,the pressure dropped to 240.9. At that time, the monomer feed to thereactor was commenced and continued for over two hours at the rate of3.3 grams per liter. The reaction mixture was held for three more hoursat 75° C. with stirring at 300 rpm with the pressure dropping to 221.6.During the holding period, samples were taken and evaluated for one-hourintervals of solids content. After the holding period, the resultantproduct was cooled and analyzed: milliequivalents of acid was 0.24;milliequivalents of base was 0.127; particle size was 7590 angstroms,and solids content was 24.02 percent.

EXAMPLE 2

This example further illustrates the vinyl chloride latex copolymercompositions of the invention.

    ______________________________________                                        Ingredient         Parts by Weight (grams)                                    ______________________________________                                        Reactor Charge                                                                Deionized water    500.0                                                      Ammonium persulfate                                                                              4.5                                                        Triton N 101.sup.1 15.5                                                       PMF Charge                                                                    Salt of acid-functional acrylic                                                                  742.5                                                      polymer.sup.2                                                                 Deionized water    1697.0                                                     Glycidyl methacrylate                                                                            16.9                                                       Vinyl Chloride Monomer (VCM) Charge                                           Vinyl chloride     742.5                                                      ______________________________________                                         .sup.1 Nonyl phenoxy polyethylene oxy ethanol (65% ethylene oxide).           .sup.2 40 percent acrylic acid; 59 percent styrene; 1 percent ethyl           acrylate (all on a weight basis).                                        

The reactor charge was charged to the reactor which vacuumed and set atan agitation rate of 275 rpm. The PMF charge was added to the reactorwhich was again subjected to vacuum at a set agitation rate of 200 rpm.335.6 grams of the PMF charge and 111.4 grams of the VCM charge wereadded to the reactor (at 270.9 rpm) at a pressure of 37.9.

The reactor (at 273.1 rpm) was pressurized at 148 psig in order tointroduce ethylene therein. The reactor charge was heated to 75° C. andheld for 20 minutes at 237 psig. The remainder of PMF charge and the VCMcharge were added to the reactor, respectively at the rates of 633.8grams per hour and 210.4 grams per hour for three hours. Upon completionof the addition, the reaction mixture was held for two hours at 75° C.with the pressure dropping with 277.2 psig to 173.7 psig.

The resultant mixture was analyzed: milliequivalents of acid was 0.226;milliequivalents of base was 1.53; particles size was 1260 angstroms andsolids content was 25.2 percent.

Curable coating composition of this invention were prepared with vinyllatex copolymer compositions described herein, melamine formaldehyderesins as curing agents and other resinous adjuncts and additivestypically found in internal can coatings. The coating compositions wereapplied with a No. 12 drawdown bar and baked at 400° F. for fourminutes. The cured coatings showed significantly reduced discoloration.

Therefore, what is claimed is:
 1. A coating composition containing avinyl halid latex copolymer composition having color stability,containing a copolymer of free-radical addition polymerizableethylenically unsaturated monomer in water, in the presence of apolymeric surfactant; the improvement comprising a copollymerizablealpha-olefin monomer selected from the group consisting of ethylene,propylene, isobutylene and butene-1, which is present in a small buteffective amount to reduce or prevent discoloration of a film of thevinyl halide latex copolymer composition upon baking at temperature ofabout 350° F. or higher.
 2. A coating composition of claim 1 whichfurther comprises a curing agent which is an aminoplast, phenoplast orisocyanate.
 3. An article of matter which is prepared by applying to asubstrate the coating composition of claim 2.